The present invention relates to a multilayered water-repellent film, and a method of forming the film on a glass substrate. The film-forming method belongs to the sol-gel process. The water-repellent coating is suitable for application, for example, to vehicular and architectural window glasses which are to be exposed to rain water.
Hitherto, there have been various proposals to provide a glass substrate with water repellency. For example, there have been proposed water-repellent agents to be applied to a glass substrate surface, each of which contains a silane compound having a polyfluoroalkyl group or a condensate of the partially hydrolyzed silane compound (see JP-A-58-122979, JP-A-58-42958, JP-A-58-147483, JP-A-58-172242, JP-A-58-172243, JP-A-58-172245, JP-A-58-172246, JP-A-58-190840 and JP-A-58-223634). JP-A-58-167448 discloses a reflectance reducing agent which contains a silane compound having a polyfluoroalkyl group or a condensate of the partially hydrolyzed silane compound.
JP-A-58-129082 and JP-A-58-172244 disclose, as water-repellent agents, silicone block oligomers each having a polyfluoroalkyl group.
JP-A-5-345641 discloses a method of forming a water-repellent film on a glass substrate. This method comprises the Steps of: (a) applying a mixture to the glass substrate and (b) baking the coated glass substrate in an atmosphere containing fluoroalkylsilane vapor so as to increase the fluoroalkyl content of the water-repellent film. This mixture contains a metal alkoxide, a substituted metal alkoxide in which alkoxyl groups are partially replaced with fluoroalkyl groups, an alcohol, water, and an acid or a base.
JP-3-90345 discloses a high refractive index dielectric layer formed on a transparent substrate and a low refractive index layer formed on the dielectric layer. This dielectric layer contains inorganic fine particles which may be tin oxide doped with antimony. The low refractive index layer is prepared from a mixture containing a fluoroalkyl-containing compound, a silicon compound, an alcohol and an acid.
However, the above publications' proposals do not provide glass substrates with water-repellent and/or reflectance reducing films which are superior in durability and weatherability tests.
There is a glass substrate coated with a film of Teflon (a trade name). However, this film is soft and thus tends to have scratches thereon. With this, the film becomes opaque.
JP-A-5-51238 discloses a water-repellent film formed on a glass substrate. This film (2) has a metal oxide phase (21) and water-repellent fine particles (22) dispersed in the metal oxide phase (21). However, this film has a problem that it tends to have scratches thereon.
JP-63-117933 discloses a method of providing a glass substrate with water repellency. This method comprises irradiating the glass substrate with at least one ion of at least one element selected from the group containing Sn and Sb. JP-A-4-160039 discloses a similar method of providing a glass substrate with water repellency. This method comprises the steps of: (a) forming a metal oxide film on the glass substrate and (b) irradiating the film's surface with at least one ion of at least one element selected from the group containing Sn and Sb for providing the film with water repellency. However, according to these two methods, the initial contact angle of water drop on the film does not become sufficiently large, and the ion will be gradually oxidized and thus water repellency does not last for a long time.
There are provided proposals to form on a glass substrate a multilayered water-repellent film having a first layer adherent to the glass substrate and a water repellent second layer formed on the first layer.
For example, JP-A-60-231442 discloses a water-repellent film having a first layer of a first polymer of a first organic silicon compound having a siloxane bond and a second layer of a second or third polymer. The second polymer is prepared from a second organic silicon compound which contains more carbon or less oxygen than that of the first organic silicon compound. The third polymer is prepared from a fluorine compound. However, this film tends to have scratches thereon.
JP-A-3-153859 discloses a water-repellent film formed on a plastic substrate. This film has a first metal oxide layer formed on the plastic substrate and a second layer of a mixture of a metal oxide and a fluorine-containing resin. However, adhesion of this film to the substrate is inferior because the substrate is plastic.
JP-A-2-311332 discloses a method of a water-repellent film on a glass substrate. This method comprising the steps of: (a) forming a metal oxide layer on the glass substrate; and (b) treating the metal oxide layer with at least one silyl compound which is selected from the group consisting of chlorosilyl compounds, alkoxysilane compounds and fluoroalkylsilane compounds. JP-A-5-238781 discloses another water-repellent film having a first layer of SiO.sub.2 and a second layer of perfluoroalkylalkylsilane. However, these water-repellent films are not sufficient in durability of water repellency in a severe environment.
To increase adhesion between the first and second layers of a water-repellent film, there have been proposed some conventional methods to make the first layer's surface minutely rough. For example, JP-A-4-124047 discloses a method of forming a water-repellent film having first and second layers. This method comprises the steps of: (a) forming the first metal oxide layer on a glass substrate; (b) etching the first layer so as to make the first layer's surface minutely rough; and (c) forming on the first layer the water repellent second layer. JP-A-6-116430 discloses another method of forming a water-repellent film having first and second layers. This method comprises the steps of: (a) forming the first layer such as SiO.sub.2, film on a plastic film; (b) etching the first layer by plasma discharge so as to make the first layer's surface minutely rough; and (c) forming the second layer (a chemical adsorption monomolecular layer containing fluorine) on the first layer through siloxane bonds. However, according to these methods, the processes to make the first layer's surface minutely rough are complicated. Furthermore, according to these methods, the configuration or roughness of the etched surface of the first layer is inferior to sufficiently increase adhesion between the first and second layers. Thus, the water-repellent films according to these methods are not satisfactory in durability of water repellency in a severe environment.
Another example is making the first layer's surface minutely rough by the thermal decomposition of an organic polymer added to a metal alkoxide solution. However, this method has the following drawback. Micro-pits which make the first layer minutely rough tend to disappear by the densification of the first layer after baking at a temperature not lower than 400.degree. C. This tends to make the first layer relatively flat.
JP-A-5-147976 discloses a method of forming on a glass substrate a metal oxide film having a minutely rough surface. This method belongs to the sol-gel process. In this method, a coating solution comprises at least two sols prepared from at least one compound selected from the group consisting of metal alkoxides and metal acetylacetonates. The at least two sols, i.e. at least two polymers thereof, have different average molecular weights. This method provides a metal oxide film which has a minutely rough surface having numerous micro-pits thereon.
JP-A-6-16455 discloses a method of forming on a glass substrate a multilayered metal oxide film having first and second metal oxide layers. In this method, a first coating solution which is similar to the coating solution of JP-A-5-147976 is used for providing the first layer with a minutely rough surface having numerous micro-pits thereon. Furthermore, a second coating solution which contains a silane compound having a fluorocarbon group is used in this method for forming on the first layer the second water-repellent layer. This method provides a multilayered metal oxide film which is durable in water repellency.
However, a recent demand has been increasing for a multilayered water-repellent film which is more superior in durability of water repellency, mechanical strength and chemical resistance even in a severe environment.